If an equilibrium system contains gaseous reactants and/or products, a change in volume will cause a change in concentration and pressure. A decrease in volume will cause the system to shift in the direction that produces moles of gas in total, whereas an increase in volume causes a shift in the direction that produces moles of gas in total.

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Explanation:

When a constraint is imposed on a reaction system in equilibrium, the equilibrium position will shift in such a way as to annul the constraint.

When the volume of a reaction system in equilibrium is decreased, the equilibrium position will shift in the direction having less total number of moles.

Similarly, when the volume is increased, the equilibrium position shifts to the direction having a greater total number of moles.

i'm pretty sure its b i might be wrong though!

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.